Electrolysis cell current efficiency with solid oxidizing agents

ABSTRACT

A FUSED SALT ELECTROLYSIS CELL BATH IS TREATED WITH A SOLID OXIDIZING AGENT, SUCH AS SODIUM CHLORATE, TO IMPROVE CURRENT EFFICIENCY OF THE FUSED SALT ELECTROLYSIS CELL.

United States Patent ABSTRACT OF THE DISCLOSURE A A fused saltelectrolysis cell bath is treated with a solid oxidizing agent, such assodium chlorate, to improve current efliciency of the fused saltelectrolysis cell.

BACKGROUND OF THE INVENTION In the operation of alkali metal fused saltcells, a fused salt mixture is electrolyzed to produce alkali metal atthe cathode and halogen gas at the anode. The anode is conventionally acylindrical graphite or carbon anode surrounded by an annular metalliccathode. A porous diaphragm is provided in the anode-cathode annularspace to assist in the separation of the products of electrolysis.

Metallic sodium is produced generally from a molten mixture of thechlorides of calcium and sodium in electrolytic cells of the Downs type(US Pat. No. 1,501,756) or in modifications of these cells. These cellsare char- Patented Jan. 26, 1971 This electrolyte yields sodiumcontaining less than 0.1% impurities and shows current efficiencies of85-89% which can be raised to 90-95% by addition of 12% sodium fluoride,However, this mixture is expensive because of its strontium compound inthat the original investment and maintenance are of a relatively highorder. Wood,

' US. Pat. 2,876,181, obtains high current efiiciency with acterized byhaving one or more bottom mounted vertically aligned cylindricalgraphite anodes each of which is projected upwardly into a separatecylindrical opening within a unitary cathode assembly. The sodium isproduced at the cathode surfaces and the chlorine at the anode surfaces.Due to the difference in densities between these products, and that ofthe molten bath, the products rise to the surface of the bath and arethen collected.

The aforementioned cells of prior practice have given good results, butunfortunately cells of this type rarely ever achieve currentefiiciencies greater than 80-85% (cf. Sodium, A.C.S. monograph 133, p.31, M Sittig, Reinhold Publishing Corporation, 1956). The currentefficiency of these cells increases as the operating temperature isreduced. However, crust formation or any bath solidification interferesseriously with economical operation so that satisfactory results can besecured only when the baths are maintained at temperatures above themelting point.

Alternate salt mixtures for use as electrolyte baths have been describedin the prior art and, although some of these mixtures have shownimproved current efficiencies, this advantage has been offset by highmaterial costs or production of sodium whose quality does not meetpresent high purity requirements and is not readily purified. Grabau,U.S. Pat. 464,097 (Dec. 1, 1891), disclosed a ternary mixture consistingof sodium chloride, another alkali metal halide and alkaline earthhalides. This mixture was stated to have a current efficiency of 95% butyielded sodium containing another alkali metal as an impurity. With thepreferred other alkali metal salt, potassium chloride, the sodium wasstated to contain 3% potassium. Seward et al., US. Pat. 841,724 (Jan.22, 1907), described a mixed salt bath containing sodium chloride,sodium fluoride and an alkali earth chloride. This bath gave sodium freeof other alkali metals but its current efficiency was not disclosed.However, neither of these baths appeared to have found commercialacceptance. More recently, Cathcart et al., US. Pat. 2,850,442, havedevised a mixture consisting of sodium chloride, barium chloride andstrontium chloride.

a lithium chloride-sodium chloride electrolyte but the sodium is statedto contain 4% lithium under exemplary operating conditions and a lithiumelectrolyte is expensive.

It is aapparent that a tremendous effort has been exerted over the pastyears to achieve greater current efficiency. In View of the magnitude ofthe sodium industry coupled with the importance of sodium as a chemicalintermediate and the rising power costs, the development of a relativelylow cost, readily operable bath of improved current efliciency is ofoutstanding industrial importance.

SUMMARY OF THE INVENTION This invention relates to a novel process forimproving the current efiiciency of an alkali metal fused saltelectrolysis cell comprising treating the molten cell bath with a solidoxidizing agent.

It has been discovered that this oxidation treatment increases thecurrent efficiency of a fused salt electrolysis cell in a cheap,eflicient and commercially applicable manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel process of thisinvention for improving the current efiiciency of an alkali metal saltelectrolysis cell comprises treating the molten salt bath of the cellwith a solid oxidizing agent selected from the group consisting ofchlorates, chlorites, nitrates, nitrites, hypochlorites and mixturesthereof. The treatment consists essentially of adding a given quantityof oxidant to a molten cell bath.

In normal commercial operations, a sodium cell may be operated for aperiod of several months before the current efficiency decreasessignificantly. At this point a solid oxidant such as sodium chlorate maybe added to the operating cell bath. In the alternative, a portion orall of the cell bath may be removed from the electrolysis cell andplaced in a separate container. The cell bath in the container ismaintained in a molten state and the solid oxidizing agent is then addedto this molten cell bath. In either situation, the oxidizing agentdecomposes rapidly at the high temperature of the cell bath and the cellbath current efliciency is increased within a short period of time.

The oxidizing agents may be selected from any of the well-known solidoxidizing agents. The preferred oxidizing agents are selected from thegroup consisting of chlorates, chlorites, nitrates, nitrites,hypochlorites and mixtures thereof, and these include sodium chlorate,sodium chlorite, sodium hypochlorite, sodium nitrite, sodium nitrate andpotassium chlorate. The preferred oxidizing agent is sodium chlorate.

The amount of oxidizing agent added to the cell bath may vary accordingto the condition of the cell bath, the desired degree of oxidation andspeed of oxidation. Generally, the oxidizing agent can comprise fromabout 0.01- 10% by weight of the cell bath. The preferred range of 0.55%by weight has been found to produce the desired degree of oxidation andimproved current efficiency. Additionally, the oxidizing agent may beadded to the cell bath in one lump portion or in incremental portions.For example, a total of 15 pounds sodium chlorate may be added in three5 pound additions to the cell bath.

The reasons for a current efiiciency recovery from the oxidationtreatment are not fully understood. While this invention is not intendedto be based upon any particular theory, it is speculated that poorcurrent efficiency may be due to the presence of small particulateelectronic conductors in the bath which serve to transfer metallicsodium to the anolyte by a concentration cell mechanism. At the anolytethe sodium is destroyed by the direct combination with chlorine. Theaddition of an oxidizing agent serves to change the particulateelectronic conductor to an inert oxide. Thus, current efficiency loss bya concentration cell mechanism cannot take place.

The following example is illustrative of the practice of this invention.The electrolyte composition is reported as percent by weight.

EXAMPLE A substantially conventional Downs cell was charged with anelectrolyte containing about 26% sodium chloride, 24% calcium chlorideand the remainder barium chloride. This bath had a melting point ofabout 560 C. and was operated with a direct current for several monthsat an average of about 605 C. This cell was equipped with a meshdiaphragm between anode and cathode and the spacing between theseelectrodes was 1.5 inches. Electrolysis was carried out at about 38,000amps. and at a cell voltage of about 7 volts.

A portion of the cell bath was transferred to a laboratory compartmentalcell where the current efficiency was found to be 73%. An identicalportion of the above described Downs cell bath was transferred to abeaker; the cell bath was maintained in the molten state. Then 0.5% byweight sodium chlorate was added to the molten cell bath. After a fewminutes the cell bath was transferred to the laboratory compartmentalcell where the current efiiciency was measured and found to be 87%. This14% increase in current efficiency is attributed to the treatment withthe oxidizing agent.

Since it is obvious that many changes and modifications can be made inthe above-described details without de- 4 parting from the nature andspirit of the invention, it is to be understood that the invention isnot to be limited to said details except as set forth in the appendedclaims.

I claim:

1. In a process for producing alkali metal by electrolyzing a fusedalkali metal salt bath and recovering said alkali metal which isliberated at the cathode, the improvement comprising regenerating a usedalkali metal fused salt bath by adding a solid oxidizing agent to saidbath.

2. In a process for producing alkali metal by electrolyzing a fusedalkali metal salt bath and recovering said alkali metal which isliberated at the cathode, the improvement comprising regenerating a usedalkali metal fused salt bath by adding 0.01-10% by weight of a solidoxidizing agent selected from the group consisting of chlorates,chlorites, nitrates, nitrites, hypochlorites and mixtures thereof tosaid bath.

3. A process in accordance with claim 2 wherein the oxidizing agent isselected from the group consisting of sodium chlorate, sodium nitrate,sodium chlorite, sodium hypochlorite, potassium chlorate, and mixturesthereof.

4. A process in accordance with claim 2 wherein the oxidizing agent issodium chlorate.

5. A process in accordance with claim 2 wherein the oxidizing agent isadded in incremental portions.

References Cited UNITED STATES PATENTS 2,148,404 2/1939 Gilbert 204683,072,544 1/1963 Kroon et al. 20468 3,265,490 8/1966 Yoshizawa et al.20468XR 3,357,904 12/1967 Steele 20468 JOHN H. MACK, Primary Examiner D.R. VALENTINE, Assistant Examiner

